1. Field of the Invention
The present invention relates to methods and apparatus for generating seismic waves and more particularly to apparatus for generating elliptical polarized seismic shear waves in a marine environment.
2. Related Prior Art
Prior art includes several methods for generating seismic waves in a marine or subsea environment. United States patents that are representative of the state of the art illustrating devices for the generation of seismic waves in marine environments are U.S. Pat. Nos. 3,365,019, (Bays) 3,394,775, (Cole, et al.), 4,014,403, (Mifsud) and 4,516,227, (Wener, et al.).
U.S. Pat. No. 3,365,019, "Seismic Vibrator for Marshlands and Submarine Use", (Bays), relates to a vibrator for marshlands use, having a cup shaped earth coupling member oriented for contacting the earth at its open end, and having a reaction mass member mounted to the coupling member. A suction device is provided for coupling the vibrator to the earth and a pressure device for releasing the coupling member.
U.S. Pat. No. 3,394,775, "Marine Vibration Transducer", (Cole, et al.), relates to a pressure-compensated acoustical wave generator having a device for slidable sealing a piston to a support member, and a flexible seal. The flexible seal is secured at one end to the outer periphery of the support member and at the other end to the outer periphery of a portion of the piston.
U.S. Pat. No. 4,014,403, "Vibratory Apparatus for Use in Seismic Exploration", (Mifsud), relates to a variable frequency seismic vibrator, including an earth coupling plate, a reaction impedance generator, and an energy source for generating reciprocating movement of the coupling plate relative to the reaction impedance. The reaction impedance is generated by a reaction mass and a spring of variable stiffness which couples the reaction mass to the vibrator. As the frequency of vibration changes, the stiffness of the spring is automatically adjusted so that the impedance of the spring resonates with the impedance of the mass to maximize the reaction impedance.
U.S. Pat. No. 4,516,227, "Subocean Bottom Explosive Seismic System", (Wener, et al.), relates to a system which provides at least one subocean bottom seismic device, such as a seismic source or a seismic detector, and a planting unit. When the planting unit is lowered, it selectively implants the seismic device at predetermined locations in the ocean bottom, it releases from the implanted seismic device, and uncoils a signal cable from the implanted seismic device when it is raised. The signal cable, which is capable of retrieving the implanted seismic device, is connected to an anchored buoy which contains a first communications unit. A second seismic device is carried in a predetermined pattern near the implanted seismic device and is connected to a second communication unit.
Prior art patents that relate to methods for acquisition of seismic data in a marine environment are represented by U.S. Pat. Nos. 3,781,775, (Malloy), 3,810,524, (Dransfield), 4,558,437, (Meeder, et al.) and 4,766,574, (Whitmore Jr. et al.).
U.S. Pat. No. 3,781,775, "Rotating Stereo Sonar Mapping and Positioning System", (Malloy), relates to a stereo sonar system including a pair of pulsed sonar transducers mounted one above the other on a stationary bottom frame. The transducers are rotated as a unit, but are spaced sufficiently to produce a three dimensional figure when stereo viewed.
U.S. Pat. No. 3,810,524, "Apparatus for Carrying a Seismic Energy Generator", (Dransfield), relates to an apparatus for carrying a seismic energy generator, which includes a frame and a carriage movable with respect to each other. The frame and carriage are interconnected by a unidirectional dashpot which allows the carriage upon which the generator is mounted to move freely upwardly, and constrains it from moving rapidly downwardly. In one embodiment, the dashpot includes a piston, with holes contained within a hydraulic cylinder and a plate mounted adjacent to the cylinder. Also included is a spring biased away from the piston to cover the holes when the bias is overcome by hydraulic fluid pressure. This is done to restrict the fluid flow and movement of the carriage in one direction.
U.S. Pat. No. 4,558,437, "Seafloor Velocity and Amplitude Measurement Apparatus and Method Therefor", (Meeder, et al.), relates to a system which provides a method and apparatus for measuring the velocity and amplitude of sound waves from acoustic pulses generated near the mud line of the seafloor. The apparatus includes a seismic source for generating the acoustic pulses, one or a plurality of vertically spaced sensors located vertically below the seismic source for sensing when the sound waves impact the sensors and a hydrophone sensor located on the vessel for measuring the distance to the seismic source. Also included are a crane and winch for pulling the embedded sensors upwardly and out from the sediments, and a device for firing the seismic source when said sensors are being pulled upwardly. Devices for taking amplitude and velocity measurements from each sensor and for determining the distance the sensors have been pulled upwardly for each acoustical pulse operation are included.
U.S. Pat. No. 4,766,574, "Method for Depth Imaging Multicomponent Seismic Data", (Whitmore Jr. et al.), relates to a method for imaging multicomponent seismic data to obtain depth images of the earth's subsurface geological structure as well as estimates of compressional and shear wave interval velocities. Measures are obtained of imparted seismic wavefields incident on reflecting interfaces in the earth's subsurface and of scattered seismic wavefields that result from wavefields incident on the interfaces. The incident and scattered seismic wavefields are employed to produce time dependent reflectivity functions representative of the reflecting interfaces. By migrating the time dependent reflectivity functions, depth images of the reflecting interfaces can be obtained. For a dyadic set of multicomponent seismic data, the dyadic set of multicomponent seismic data are partitioned so as to separate the variously coupled incident and reflected wavefields in the recorded multicomponent seismic data. The incident and reflected wavefields are cross correlated to form time dependent reflectivity functions. The time dependent reflectivity functions are then iteratively migrated according to a model of wavefield velocities of propagation to obtain estimates of the compressional and shear wave interval velocity. The migrated reflectivity functions can then be stacked to produce depth images of the earth's subsurface geological structures.